Telnet Protocol-a proposed document(2)

　　The code of the NVT will be full ASCII. The seven-bit code will be　transmitted in eight-bit bytes, the high order bit set to zero.

　　It will be the responsibility of the using site to provide its users　with a means of producing all 128 ASCII codes, as well as a selected　set of special TELNET control signals (see Figure 3).

　　The ASCII character ESC will be employed by the user as an escape　signal indicating that the next character(s) has special meaning.

　　The meaning assigned to escape code will be serving site defined and　therefore may not be consistant across the network.

　　It will be the responsibility of the serving site to specify for　users how the NVT code will be used to represent the codes normally　generated by a local terminal. The serving sites specification of　this representation is expected, where reasonable, to map on a one-for-one basis for ASCII graphics and controls that are provided　through local terminals. The serving site will also specify how the　escape conventions will be interpreted by the system.

　　The end of a line will be represented in the NVT as carriage return　followed by line feed.

　　The protocol assumes that at initially the serving site will not　provide any echo to the using site.

　　Each TELNET control signal for which code must be sent over the　connection will be represented in the NVT by an eight-bit code, with　the high order bit set to one. Following are the special codes　established to date. (U) indicates that in most implementations the　user would be expected to have the ability to signal the TELNET　process from his terminal to initiate the code.

　　Code X'A0'

　　Source: Both Sites (U)

　　Meaning: A DATA TYPE[1] signal indicating that code will be　transmitted by NVT, i.e., using the seven-bit ASCII　conventions.

　　Code X'80'

　　Source: Using Site (U)

　　Meaning: Order using site NCP to send an INS and insert X'80' in　data stream.

　　Code X'81'

　　Source: Using Site (U)

　　Meaning: Break or Attention

　　Code X'82'

　　Source: Serving System

　　Meaning: Reverse Break

　　Code X'83'

　　Source: Both Sites

　　Meaning: I Echo

　　Code X'84'

　　Source: Both Sites

　　Meaning: You Echo

　　Code X'85'

　　Reserved

　　Code X'86'

　　Reserved

　　Code X'87'

　　Source: Both Sites

　　Meaning: This site has not implemented the following control code.

　　Code X'88'

　　Source: Both Sites

　　Meaning: [2]Return to control mode, i.e. next byte will be a　control signal, possible a new DATA TYPE.

　　Some special TELNET control signals are required to permit the user　on some systems to send control information to the using site TELNET　process. These do not require a corresponding control code for　transmission. The local TELNET control signals are:

　　1. Transmit all data to this point.

　　2. Suppress transmission of end of line, send all other data.

　　Data is to be forwarded to the NCP for transmission as convenient,　but at least at the end of line, end of line suppression, and　transmit signals. If the normal line length of the sending site is　greater than the allocation given by the receiving site, the sending　sites TELNET process or TELNET server will be responsible for　breaking the line into convenient lengths and turning them over to　the NCP for transmission.

　　This document will be revised as necessary to provide conventions for　data types, in addition to the NVT ASCII type.

　　Footnotes:

　　[1] A one-byte DATA TYPE signal is sent as the first byte of data　over a connection. A default is employed if the first byte over a　connection has the high order bit set to zero, and it is assumed that　the seven-bit ASCII NVT convention will be employed. After initial　connection, the DATA TYPE may be changed (see code X'88'). Most　implementations and applications may expect the DATA TYPES to be　symmetrical at any point in time,(i.e. both using a serving site　using the same DATA TYPE.).

　　[2] A site receiving a DATA TYPE signal is to respond with a double　X'88' if the new DATA TYPE is acceptable.

　　|<------- 32 ------->|<-8->|<-8->|<-- 16 -->|<-8->|<---

　　+--------------------+-----+-----+----------+-----+------------------

　　| leader | x |size | count | x | TEXT

　　+--------------------+-----+-----+----------+-----+------------------

　　|<---- level 1 ---->|

　　message leader

　　|<------------------ level 2 ------------------>|

　　message preamble

　　level 3

　　|<- message text..-->

　　Figure 1. Network Message on Link 2-31

　　Indicating Portions of Interest to Various Levels

　　USING HOST Serving HOST